1. Field of the Invention
The invention relates generally to an internal combustion engine and a method for controlling an internal combustion engine. More specifically, the invention relates to an internal combustion engine that has a circulation path formed by connecting an exhaust passage and an intake passage to each other, and a method for controlling the internal combustion engine.
2. Description of the Related Art
Existing internal combustion engines are operated with the use of the pressure that is generated when fuel is burned in combustion chambers. There is an internal combustion engine that has a circulation path formed by connecting an exhaust passage, through which exhaust gas generated after combustion flows, and an intake passage, through which gas that is taken into a combustion chamber flows, to each other. In such an internal combustion engine, for example, hydrogen, which is used as fuel, and oxygen, which oxidizes the hydrogen, are supplied to the combustion chamber, and argon, which is used as working gas having a specific heat ratio higher than that of air, is circulated back to the combustion chamber through the circulation path. Thus, when the internal combustion engine is operated by burning the hydrogen in the combustion chamber, thermal expansion of the argon, which takes place in the combustion chamber, is also used to operate a piston. Therefore, thermal efficiency during operation of the internal combustion engine is improved. When hydrogen is burned, water is formed. However, the water is condensed and separated from the argon in a condenser provided in the circulation path. As a result, only the argon is circulated back to the combustion chamber.
However, in such an internal combustion engine, the thermal efficiency varies depending on the argon concentration in the gas taken in the combustion chamber. Therefore, the torque required when the internal combustion engine is operated is not obtained in some cases. Accordingly, some existing internal combustion engines having a circulation path are designed in such a manner that appropriate thermal efficiency is achieved. For example, in an internal combustion engine described in Japanese Patent Application Publication No. 2007-77834 (JP-A-2007-77834), argon, in addition to hydrogen and oxygen, is supplied to a combustion chamber based on the required torque. Thus, the argon concentration is brought to a value at which appropriate thermal efficiency is achieved. As a result, the required torque is obtained.
However, gases are not well-mixed as compared to the case where gas and liquid are mixed. Accordingly, as in the internal combustion engine described in JP-A-2007-77834, when gas, for example, hydrogen is used as the fuel that is burned in the combustion chamber, the hydrogen is not mixed well with oxygen that is used as an oxidant. Especially, when the internal combustion engine is being started, the gas is not turbulent enough. Therefore, the hydrogen and the oxygen tend to be mixed more poorly, which may deteriorate the engine startability.